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Employee Future Benefits - Summary of Disaggregation of Defined Benefit Obligation (Detail) - CAD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Pension defined benefit plans [member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation $ 3,789 $ 3,794
Retiree welfare plans [Member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation 450 466
U.S. plans [Member] | Pension defined benefit plans [member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation 2,433 2,515
U.S. plans [Member] | Pension defined benefit plans [member] | Active [member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation 526 509
U.S. plans [Member] | Pension defined benefit plans [member] | Inactive and retired [Member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation 1,907 2,006
U.S. plans [Member] | Retiree welfare plans [Member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation 336 355
U.S. plans [Member] | Retiree welfare plans [Member] | Active [member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation 9 11
U.S. plans [Member] | Retiree welfare plans [Member] | Inactive and retired [Member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation 327 344
Canadian plans [Member] | Pension defined benefit plans [member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation 1,356 1,279
Canadian plans [Member] | Pension defined benefit plans [member] | Active [member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation 116 125
Canadian plans [Member] | Pension defined benefit plans [member] | Inactive and retired [Member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation 1,240 1,154
Canadian plans [Member] | Retiree welfare plans [Member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation 114 111
Canadian plans [Member] | Retiree welfare plans [Member] | Active [member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation 0 0
Canadian plans [Member] | Retiree welfare plans [Member] | Inactive and retired [Member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation $ 114 $ 111